(651b) The Superposition of Dielectric and Donnan Exclusion and Their Influence on Sodium Chloride Transport in Sulfonated Polysulfone

Polymers have been used as membranes that can desalinate saline water (via processes such as reverse osmosis (RO)) to address global water shortages. While current state-of-the-art polyamide membranes exhibit favorable desalination transport properties, amide linkages along the polymer backbone are susceptible to oxidative degradation caused by chlorine-based compounds. Sulfonated polysulfones are promising desalination materials with resistance to such oxidative degradation issues, but these materials have not yet achieved the same level of water/salt transport selectivity as current state-of-the-art polyamide materials.

One technique to increase the water/salt transport selectivity is to use molecular engineering strategies that improve the polymer’s ability to exclude salt (i.e., reduce the salt partition coefficient). Previous reports suggest that salt partition coefficients may be reduced by leveraging dielectric exclusion (which results from ions experiencing a dielectric constant discontinuity at the solution/polymer interface) and Donnan exclusion (which results from the incorporation of fixed charges along the polymer backbone). It is unclear, however, if both mechanisms can be effective simultaneously in a given polymer. To understand the contributions of both dielectric and Donnan exclusion phenomena to salt exclusion, we structurally modified two series of sulfonated polysulfones to control the fixed charge concentrations and dielectric constants of the polymers.

Overall, the superposition of dielectric and Donnan exclusion effects reduces the salt partition coefficients of hydrated polymers more than either mechanism alone. Moreover, we propose an analytical framework that describes the influence that the superposition of both exclusion mechanisms has on the overall salt transport properties. These results provide deeper insight into how exclusion mechanisms influence the salt transport properties in hydrated charged polymers, which may be helpful to inform efforts to engineer advanced polymers for desalination applications.